"DonMorrisey" wrote in
oups.com:

A friend's homebuilt recently went down with an 8000 hour pilot at the
controls. No one was hurt and the plane was landed in a field. The
pilot reported hearing/feeling an extremely loud vibration from the
wing, he immediately killed power and landed. One of the wings was
visibly buckled. The airplane is all aluminum with a 36 ' wing span.
The NTSB showed up and after getting the plane to a hangar, the wing
was dismantled. The rear spar had failed and was twisted and buckled,
although the wing had held together. The NTSB guys said the very
solid structural design and construction of the wing was what saved
the plane from coming apart in the air, as well as the pilot's quick
response. The conclusion by the NTSB was aileron flutter, they found
several bolts in the aileron hinge bracket assemblies (which hold the
aileron to the rear spar) that were supposed to have been drilled 10
MM from the top and bottom edge of the spar flange and these had been
drilled at 25 MM or so. That was it and they walked away. There was
no visible damage to the aileron, the aileron spar or the hinge
bracket assemblies. The pliot reported no vibration in the stick. My
friend is convinced it had something to do with the washout of the
wing. The wing is a NACA 4413 Airfoil, modified in that it has no
undercamber on the bottom, the bottom is flat. The washout, which is
built into the wing during construction, is a couple of degrees. The
plane I am building uses the same wing/washout. I have done research
but can't find anything specific linking washout to flutter.

Any thoughts on this??


Umm, unlikely. Mostly flutter is caused by the surface itself deflecting
whatever it's been attached to and when that surface moves the related
control surface moves with it and inertial forces cuase it to deflect in
the opposite direction, loading up the surface. In one scenario, for
instance, the wing is deflected rapidly upwards by a gust. the aileron
is left behind because of inertia, i.e, the trailing edge of the
aileron's mass tends to deflect it downwards which of course now adds to
the wing's journey upwards. the wing loads up, bends, and eventualy
springs back in the other direction, down, thereby causing the aileron
to now deflect upwards and so on. the mass balance attached to a control
surface should eliminate this problem. Flutter can also be induced by an
aerodynamic cyclic where the flow covering the surface is cycled. I've
seen this and it's nowhere near as destructive as the previous example.
One thing worth remembering about flutter is that the tendency of a
surface to flutter is exacerbated by the load on the wing. That is to
say that a surface is more likely to flutter if it's loaded up with G
than it is if not. So, you screw up a maneuver and end up pointing
downhill and accelerating fast. Off comes the power and up comes the
nose and the G. This is a likely scenario for flutter.. Anything that
loads a susceptable surface up is going to induce it more quickly at a
given speed than the same scenario without the extra G, whether it be
caused by manueveing, turbulence or W.H.Y..

Needless to say, a control surface which has been incorrectly attached
to the airframe is significantly more likely to suffer.

Even if a control surface has been correctly balanced during building,
care must be exercised to ensure balance is maintained after painting.
Paint can add significantly to the weight and quite a few accidents have
been attributed to neglecting to do this..